For many years refrigeration apparatus has been used for both heating and cooling of a space such as in a home. The apparatus is commonly known as refrigeration "heat pump" apparatus. With the use of a heat pump, the advantages diminish as the outdoor temperature drops; in fact, at low outdoor temperatures the capacity of the heat pump may not be sufficient to maintain the space temperature required in the house. The use of auxiliary heating apparatus such as electric strip heaters is quite common; however, with the increase in the cost of electrical energy, other auxiliary heating sources may be desired, such as a gas furnace.
When a gas furnace is used as an auxiliary heating source and is located upstream of the heat pump, care must be taken to ensure that the refrigeration heat pump is not operated when the furnace has a high temperature output. Some prior art systems of the type disclosed in the M. G. Garst, et al, U.S. Pat. No. 3,996,998, issued Dec. 14, 1976, make use of time delay devices or temperature responsive devices which are used to prevent the operation of the heat pump when the plenum temperature of the furnace is above a predetermined temperature. Thus, when the furnace is on, the heat pump is off and, after the operation of the furnace is terminated, either a time delay is imposed on the system before the heat pump can be operated, or a temperature responsive device is used to prevent the heat pump from operating until the plenum temperature is reduced. Both of these prior art concepts add cost to a control system.
Another prior art control system has a multistage space thermostat for controlling a heat pump and an auxiliary heating source or furnace for interposing a time delay in the operation of the heat pump after the furnace has operated. The delay is accomplished by transferring the operation of the auxiliary heating furnace to the first stage of the space thermostat and not allowing the heat pump to be energized until the first stage is satisfied. The heat pump is delayed in its operation until the space temperature increases through the first stage differential temperature. This delay is sufficient time for the temperature of the furnace to be reduced to an acceptable operating temperature for the heat pump.
In such a control system large temperature swings of the space are common along with overshoot of the temperature setting of the first stage, both adding to discomfort.
The present invention is concerned with an improvement to a control system having a two stage thermostat with anticipation for each stage whereby when the second stage calls for heat by the furnace, the heat pump is deenergized and the furnace is connected to the first stage. To lessen temperature swings and overshoot of the space temperature, the anticipation of both stages are controlled by the first stage to increase the anticipation to the thermostat.